Cathode for electron-emitting devices



Oct. 8, 1929. 5, s b e 1,730,837

CATHODE FOR ELECTRON EMITTING DEVICES Filed Aug. 31, 1928 ,EYfeYZZarX Patented o... s, 1929 UNITED STATES PATENT OFFICE FREDERICK S. ARMSTRONG, OF RIVER FOREST, ILLINOIS, ASSIGNOR TO NATIONAL UNION RADIO CORPORATION, A CORPORATION OF DELAWARE CATHODE FOR ELECTRON-EMITTING DEVICES Application filed August 31, 1928. Serial No. 303,201.

Cathodes for thermionic tubes of the A. C.v type at the present time contain cathodes in which a thin metal sleeve, carrying a negative potential, surrounds the refractory heating element and in turn is coated with electron-emitting material such as, for example, barium oxide or strontium oxide. Heat is supplied to the heating element by means of an energized wire extending through the same. The heat does not pass directly from the heating element to the coating, in these old forms, but the metal sleeve must first be heated and the heat be then transmitted from the sleeve to the coating. Furthermore, the coatings are unstable, and, in these old forms, deteriorate rapidly in service. The coatings have heretofore been applied in a plastic form and then baked. The baking operation results in the trapping of gases in the cathodes, and the subsequent elimination of such gases is quite difficult.

The object of the present invention is to produce a cathode of the general type referredto above, which in its construction and process of manufacture, shall be free from the objections above mentioned.

Viewed in one of its aspects, the present invention may be said to have for its object, a cathode construction that will insure long life. Viewed in another of its aspects my invention may be said to have for its object to produce a cathode that will be more rugged and less susceptible to damage than are cathodes as heretofore constructed. Viewed in another of its aspects the present invention may be said to have for its object a cathode that may be more easily manufactured than heretofore, at the same time preventing the trapping of gases that must afterwards be eliminated. Viewed in still anotheraspect, my invention may be said to have for its object to produce a cathode that will require less current than heretofore, for heating the same.

In carrying out my invention, I place the electron-emitting material directly upon the heating element so that the heat is transmitted directly from the latter to the former without passing through some intervening medium. The metal element adapted to carry the negative potential is in the form of a member in intimate contact with the coating material and serving mechanically to reinforce the same and hold it in place, while at the same time leaving more or less of the surface of the coating exposed. The metal element may take any suitable form, being conveniently fine wire mesh bent into the form of a sleeve or tube fitting around the heating element. The coating material may be applied to the device after the perforated sleeve or tube has been placed in position, in much the same way that plaster is ap plied to expanded metal lath. The coating material enters the interstices in the metal sleeve, and where the latter consists of ,wire mesh, fills the cavities on the inner or rear side, so that the sleeve is in effect embedded in the coating. In this way there may be obtained a coating much thicker than has heretofore been possible. Therefore, the life of the device, in service, will be greatly lengthened.

Because the coating is reinforced if interlocked with the perforated sleeve, baking of the coating is unnecessary. notwithstanding its comparatively great thickness. Therefore, the device may be treated in a fully assembled condition, before the coating is applied, to degas the same. Then, when the coating is applied, the device is ready for use without first undergoing the treatments that have heretofore been necessary for the purbaking process.

The various features of my novelty whereby my invention is characterized will hereinafter be pointed out with particularity in the claims: but, for a full understanding, of my invention, and of its objects and advantages, reference may be had to the following detailed description taken in connection with the accompanying drawing, wherein:

Figure 1 is a view on a greatly enlarged scale, partly in side elevation and partly in section, with portions of successive layers broken away, of a cathode embodying my invention; Figure 2 is a transverse section through the device, on a still larger scale; and Figure 3 is a vertical section taken on a radial plane through the device showing a fragment on a still larger scale.

Referring to the drawing, 1 represents the heating element of any usual or suitable form,

5 and of any suitable material that is a conductor of heat, and a non-conductor of electricity.

Ordinarily this element Will be in the form of a long cylinder and will be made of refractory material such as quartz or the like. Heat is supplied, as heretofore, by a current flowing through the fine Wire 2 extending lengthwise, back and forth, through the same.

In the particular embodiment of my invention illustrated the number 1 is enclosed Within the sleeve 3 formed of Wire mesh and having a suitable terminal 4 projecting from one end thereof. Plastered over the sleeve is a layer of suitable electron-emitting material.

It Will be seen that the. coating material extends through the openings in the sleeve and comes into intimate contact with the body of refractory material so as to take up heat directly fromthe same. Furthermore, the coating material enters behind portions of the Wires so as to fill all of the cavities on the inner side of the sleeve. The result is that the sleeve is embedded in a'thick coating layer so that the latter is in intimate contact with both the sleeve and the heating element. The sleeve that serves to carry the potential that causes emission from the coating. reinforces the coating mechanically and holds the coating firmly in place.

While I have illustrated and described my invention as applied to a thermionic tube. I do not desire to be limited to this particular use, since it may be employed as a cathode in any electron-emitting device. Furthermore, I do not desire to be limited to the particular details of construction illustrated and described; but intend to cover all forms and arrangements coming Within the definition of my invention constituting the appended claims.

I claim:

1. A cathode comprising an electrically non-conductive heating element. an electronemitting coating on and in direct contact with said element and perforated metal means holding said coating in place.

2. A cathode comprising an electrically non-conductive heating element, an electronemitting coating on and in direct contact with said element and perforated means of electrically conductive material coextensive with said coating and holding it in place.

3. A cathode comprising a electrically nonconductive heating element, an electronemitting coating on and in direct contact with said element, and perforated means of electrically conductive material embedded in said coating. I

4. A cathode comprising an electrically non-conductive heating element, an electronemitting coating on said element, and a Wire mesh embedded in said coating.

5. A cathode comprising a refractory heating element, a shell of wire mesh surrounding said element, and a coating of electron-emitting material over said Wire mesh and extending through the same into contact with the heating element.

6. A cathode comprising an electrically nonconductive heating element, an electron emitting coating on said element, and me- 1 holding means in contact With said coating and having interstices through which the coating material is exposed.

In testimony whereof, I sign this specification.

FREDERICK S. ARMSTRONG. 

